1. Field of the Invention
This invention pertains generally to magnetic random access memory (MRAM), and more particularly to voltage controlled MeRAM using magneto-electric tunnel junctions (MEJs) for providing burst writing and back-to-back reads.
2. Description of Related Art
The electronics industry continually seeks higher density memory devices that operate with lower power consumption since electronic devices increasingly include significant amounts of solid state memory.
Dynamic random access memories (DRAM) are dense, but volatile in that they do not retain data for any significant amount of time without refreshing (a form of rewriting). DRAMs also consume large amounts of power. Static RAM (SRAM) memories are fast, yet they are large and consume significant power. FLASH memories are a form of electronically erasable programmable read only memory (EEPROM), and while being very dense and nonvolatile, provide very slow accesses and have a high power consumption.
One class of memory devices drawing increasing attention as a next generation memory type are magnetic memory devices (MRAM) which are inherently non-volatile.
Magnetic random access memory (MRAM) has generally been implemented with a magnetic-field-controlled or electric-current-controlled write mechanism. Data within an MRAM is not stored as electric charge or current flows, as in a conventional RAM, but instead by magnetic storage elements formed from ferromagnetic plates, separated by insulating material. In a magnetoresistive RAM, reading is performed in response to measuring the electrical resistance of the cell, which changes due to the orientation of the fields in the two plates. Data writes are performed by inducing a magnetic field in response to current through write lines to change magnetic orientation.
One form of magnetic memories are spin transfer torque RAM (STT-RAM) memories. Spin transfer torque (STT) techniques use spin-aligned (“polarized”) electrons to directly torque the domains. In particular, if the electrons flowing into a layer are forced to change their spin, this will develop a torque that will be transferred to the nearby layer. Using STT, power requirements are substantially lowered. There has been a significant amount of research and development on STT-RAM, where electric currents are driven through a magnetic tunnel junction (MTJ) bit to switch it and thus to write information into it. The use of currents for writing STT-RAM, however, still involve substantial energy dissipation, and allow only a limited memory array density since each magnetic bit requires a large access transistor to drive its large write current, which also limits its scalability. Furthermore, STT-RAM memories also suffer from possible disturbance of the stored information during read operation.
A prior patent application of the Applicant describes a voltage-controlled (i.e., electric-field-controlled) magneto-electric random access memory (MeRAM), where information is written into the bit using pulsed voltages, rather than currents. MeRAM is nonvolatile, very dense, fast, and extremely low power. Other memory technologies (DRAM, SRAM, FLASH, STT-RAM) have a performance-reliability trade-off associated with bit readout, which is eliminated using MeRAM. MeRAM also offers a superior scaling behavior for energy and density, compared to STT-RAM, due to the fact that it uses voltages, rather than currents, to write information into the memory bits. MeRAM can provide high density memories with low power, yet it would be beneficial to increase the speed of read and write operations on these devices while eliminating the possibility of data being disturbed during read operations.
Accordingly, a need exists for energy efficient, high density magnetic memory devices that provide fast access and that are not subject to read disturbance. The present invention fulfills these needs and others while overcoming many shortcomings of previous magnetic memory devices.